Process and composition for sensitizing articles for metallization

ABSTRACT

Non-metallic articles are sensitized for deposition of adherent metal from electroless metal solutions in contact therewith by treatment with a first medium including a compound or combination of compounds containing an element which in one of its states is catalytically active to electroless metal deposition thereby adsorbing on the surface in situ the elemnt(s) and/or compound(s); thereafter treating the surface with a second medium to decrease the solubility of the compound or combination of compounds and/or simultaneously removing the excess; and simultaneously or subsequently, if necessary, treating the surface with one or more agents to transfer said element(s) into a catalytically-active state.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of Ser. No. 632,700, filed Nov. 17,1975, abandoned which is a continuation of Ser. No. 407,555, filed Oct.18, 1973 (now abandoned), which, in turn, is a continuation-in-part ofapplication Ser. No. 270,861, filed July 11, 1972 (now abandoned).

This invention relates to processes and compositions for sensitizingarticles to the deposition of metals from solutions thereof. Moreparticularly, it relates to improved means to provide adherent metallayers on non-metallic articles by treating such articles with a seriesof media which deposit catalytically active elements or precursors andcontacting with electroless metal deposition solutions.

BACKGROUND OF THE INVENTION

Sensitizing non-metallic substrates to the deposition of electrolessmetal, e.g., Group IB and VIII metals, i.e., copper, cobalt, nickel,gold, silver and the like, is a key step in the production of decorativeand industrially useful metallized objects, such as name plates, dials,printed circuits, and the like. This sensitization is conventionallycarried out by treating the substrate either stepwise with preciousmetal, e.g., palladium or platinum, ions in solution followed by asolution of stannous tin or similar ions, or all in one step with aunitary colloidal suspension of precious metal or with a soluble complexof precious metal ion, stannous ion and an anion. This produces asensitive surface which when immersed in a conventional electrolessmetal deposition bath causes metal to deposit on all of the sensitizedareas thereof.

A number of proposals have been made to carry out such processes moreeconomically and efficiently:

Chiecchi, U.S. Pat. No. 3,379,556 discloses immersion in a betaresorcylato chromic chloride solution to eliminate pretreatments such assealing, sandblasting, etching and the like. This method still requiresthe use of a two step, stannous-palladium subsequent treatment, see, forexample, Schneble, Jr. et al, U.S. Pat. No. 3,403,035 and U.S. Pat. No.3,033,703. Moreover, the Werner-type chromium complexes are difficult toprepare, stabilize and use. In addition, the complex must be polymerizedafter application and before subsequent treatment steps.

Bernhardt et al, U.S. Pat. No. 3,547,784 disclose treating anon-metallic surface with stannous salt then with a silver salt and thenelectrolessly plating using processes and deposition baths for copper,nickel and silver found, for example, in Schneble, Jr. et al, U.S. Pat.Nos. 3,527,215 and 3,347,724. The Bernhardt et al process isconventional and the point of novelty resides in using a particularcopolymer of vinyl chloride, which was not easy to metallize up untilthe time of the invention.

In a more recent development, there have been provided the so-calledmetal reduction sensitizers, which can employ base metal ions, followedby treatment with reducing solutions or radiant energy, e.g., heat,light and the like, to produce the sensitized surface.

The metal reduction sensitizing process consists of coating a surface,preferably one which has been activated in known ways either to renderit permanently polarized and wettable, or microporous, with a reduciblemetal salt solution, e.g., CuSO₄.5H₂ O, NiSO₄.6H₂ O, and the like, theneither draining, semi-drying or completely drying the so-treatedsurface. Sensitization is then completed by immersing the surface into astrongly reducing medium, e.g., a sodium borohydride solution, duringwhich step the metal salts are reduced to elemental metal particles.This sensitized surface is then rinsed and electrolessly plated.

Because thorough rinsing of excess metal salts before transferring theworkpiece to the reducing medium cannot be practiced, there arenumberous difficulties when dealing with copper clad surfaces (on otherportions of the article). Rinsing usually removes all of the metalsalts. Moreover, there is also the problem of drag-over of excess metalsalts into the reducing medium, and this shortens its life and alsoturns it black with atomic metal particles.

Obviously, if a means could be provided to rinse excess and unwantedmetal salts from the surface before immersion in the reducing medium,the above noted problems would be avoided. In addition, control will befacilitated because rinsing will provide a positive indication that onlythose final catalytic elemental particles which remain are those whichare adsorbed by the surface.

According to the present invention, improvements are provided in thewettability and adsorptivity of metal compound media which are used torender surfaces sensitive to electroless metal depositions. Because oftheir improved wettability and adsorptivity, such compositions can beformulated from base metals or combinations thereof with precious metalswhich have performance comparable to traditional expensive and somewhatunstable entirely precious metal-based sensitizers.

In comparison with the prior art techniques, the instant system providesthe following distinct advantages:

(i) more complete rinsing between the first medium and any subsequenttransformation agent can now be tolerated because of tremendouslyimproved adsorption of any metallic compound or element in the firstmedium to the surface;

(ii) "take" or coverage in the electroless metal bath is wholly uniformand rapid; and

(iii) in the case of activated substrates, metallization within thesurface micropores is deep and complete enhancing bond strength.

DESCRIPTION OF THE INVENTION

According to the present invention, there is provided a process forsensitizing a non-metallic article for the deposition of adherent metalfrom an electroless metal deposition solution in contact therewith, saidprocess comprising:

(i) treating the surface or selected areas of the surface of saidarticle with a first medium comprising a compound or combination ofcompounds containing an element which in one of its states iscatalytically active to electroless metal deposition or a combination ofsuch elements thereby adsorbing on said surface in situ the said elementor elements, or a compound or compounds containing said element orelements;

(ii) thereafter treating said article with a second medium to decreasethe solubility of said compound or combination of compounds and/orsimultaneously removing unadsorbed and excess components of said firstmedium from said surface; and

(iii) simultaneously with or subsequently to step (ii) treating saidsurface with one or more agents, if necessary, to transfer said elementor combination of elements into a state which is catalytically activefor the deposition of electroless metal.

In a preferred feature of the invention, the agent or agents in step(iii) will be reducing agents, such as borohydrides, e.g., alkali metalborohydrides, amine boranes, e.g., dimethylamine boranes, hydrazinehydrate, and others. If difficulty metallizable substrates are used,e.g., acrylonitrile-butadienestyrene (ABS) polymers, in preferredembodiments step (iii) will be carried out in two separate stages andemploying two separate reducing agents. In one such process, the firstreducing agent will be a so-called secondary reducer, e.g.,formaldehyde, a formaldehyde precursor, dimethyl hydantoin, a stabilizedform thereof, and the like, and the second reducing agent will be astrong, primary reducer, e.g., a borohydride or an amine borane.

Among the features of this invention are processes wherein the elementor combination of elements are base metal elements, preferably, copper,nickel, cobalt, iron or mixtures of any of the foregoing. Under certaincircumstances, molybdenum, vanadium, and lead or mixtures thereof withany of the preferable elements cited above may also be used.

Preferably, the compound or combination of compounds in the first mediumwill be a wetting agent, i.e., it will seek and affix itself firmly tothe surface being treated, e.g., by electrical attraction or othermeans. Preferably also, the first or second medium will comprise awetting agent having a polarity which is opposite to the polarity of atleast some of the surface sites of the article to be sensitized.

The present invention also contemplates the novel compositions havingthe required characteristics, e.g., media including compounds orcombinations which are adsorbable, transformable, if necessary, andremovable in excess by a second medium, as set forth above.

By way of further illustration, but with no intention of being bound byany hypothesis, in one embodiment, a first medium is formed when acopper compound or a nickel compound or mixture thereof is mixed withammonia or amine to form a copper or nickel complex with ammonia oramine or a mixture thereof. Not only are ammonia or amines in their ownrights powerful wetting agents, but so are the formed ion complexes. Itappears that such ion complexes behave much like quaternary ammoniumcomplexes, i.e., cationically. Such positively charged (polar) ioncomplexes are adsorbed by negative surface sites on the article to besensitized.

In another embodiment a dichlorocuprate [I⁻ ] ion complex:

    [CuCl.sub.2.sup.- ] H.sup.+

is formed by boiling copper chips with cupric chloride in a large excessof concentrated hydrochloric acid. As in the ammonia complex case, thiscompound is highly polar (polarity reversed however) and readilyattaches to positive surface sites due to its relatively negativecharge.

In a further embodiment a complex metal salt, e.g., ferrous sulfate(FeSO₄) is dissolved in ammonium hydroxide and this is put onto thesurface to be sensitized.

In these embodiments, the rinse of the base with a second medium willdecrease the solubility of the compound or combination of compounds. Inthe metal ammonium complex case, the basic metal ammonium compound isformed; in the cuprous ion complex case, loss of HCl in the rinse willlead to decreased solubility. In the soluble iron salt case, the secondmedium rinse causes decreased solubility.

All three concepts can use a variety of metallic compounds.

The media can comprise organic and inorganic solvents such as water,alcohols, e.g., methanol and the like, ketones, e.g., acetone, methylethyl ketone, etc., esters, e.g., ethyl acetate. Preferably, aqueousmedia are used.

Media based on organic systems can be employed, particularly on resinbearing systems such as epoxy. In one such system, cupric chloride isreadily dissolved and complexed with dimethyl formamide (DMF). A resinbearing surface brought into contact with this solution quickly adsorbsmetal complex. The surface is then rinsed with a second medium, e.g.,water, which removes excess metal complex (and first medium) and thenthe surface is next treated with an agent to convert the metal to acatalytically active state, such as a solution of sodium borohydride orhydrazine hydrate dissolved in a suitable solvent, such as DMF.

The present invention can be used to sensitize a wide variety ofnon-metallic substrates, such as plastic, e.g., polyepoxides, phenolics,polystyrene, polyesters, nylons, polyacetals, polycarbonates, and thelike, or glass, porcelain, cloth, paper, compressed wood, and the like.ABS resins are especially efficiently metallized with a "two-stage"treatment with reducing agents, as described above and exemplifiedhereinafter. The resinous substrates can be treated in known ways toactivate the surfaces prior to sensitization--this gives the highestadhesion between subsequently deposited electroless metal and theresinous substrates.

One way to activate resinous bases is to render them permanently polarand wettable by treatment first with a pre-activating agent, e.g.,dimethyl formamide, dimethyl sulfoxide, methyl ethyl ketone or mixtureof toluene and water, etc., depending on the nature of the resin, thenwith an activator such as chromic acid-sulfuric acid, and then with areducing agent, such as sodium bisulfite or hydroxylamine hydrochloride,the result of which is to produce a permanently polarized, wettablesurface.

Such techniques are disclosed in greater detail, for example, incopending U.S. patent application Ser. No. 227,678, filed Feb. 18, 1972,the disclosure of which is incorporated herein by reference.

On the other hand, the surface of the resinous article can be partiallydegradable, or be provided with a surface layer having such properties,or contain degradable particles, such as rubber particles, and ontreatment with suitable agents, such as chromic acid, or permanganate,is caused to become microporous and thus activated to adherent metaldeposits.

Any conventional electroless metal deposition bath useful withconventionally precious metal sensitized surfaces can be used to depositmetal on the surfaces sensitized according to this invention. Generally,the deposition baths will contain an ion of the metal or metals whosedeposition is desired, a complexing agent for the ion, a reducing agentfor the ion and an agent to adjust the bath to an optimum, predeterminedpH. Such baths are amply described in the patent and textbookliterature. See, for example, the patents cited hereinabove.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following examples illustrate the processes of this invention. Theyare not to be construed as limiting.

EXAMPLE 1

An epoxy-glass laminate is provided with holes at pre-selectedlocations. The surface is rendered permanently polarized and wettable byimmersion in dimethyl formamide at 80° F. for 2-5 minutes, rinsing inwater, immersing in a solution of 100 g./l. CrO₃, 250 ml./l. ofconcentrated sulfuric acid, balance water, for about 1 minute, immersingin a 5% solution of NaHSO₃ for about 2 minutes, rinsing thoroughly incold water, then in hot (160° F.) water and air or oven-drying.

Then the laminate is reverse-masked with a conventional backgroundresist and then sensitized by immersion in and adsorption of a coppercompound from a first medium comprising

    ______________________________________                                        CuCl.sub.2.2H.sub.2 O 34     g./l.                                            Hydrochloric acid (37%)                                                                             250    ml./l.                                           NaH.sub.2 PO.sub.2.2H.sub.2 O                                                                       30     g./l.                                            Water (to make).                                                              ______________________________________                                    

The so-treated laminate is next rinsed thoroughly with a second mediumcomprising cold water to decrease the solubility of said copper compoundand to remove the excess.

The laminate is next immersed in a water or methanol solution of 1gram/liter of sodium borohydride, causing the adsorbed copper compoundto be reduced to catalytic sites of elemental copper.

The sensitized laminate is transferred to an electroless copperdeposition bath of the formula:

    ______________________________________                                        CuSO.sub.4.5H.sub.2 O                                                                            30       g./l.                                             Rochelle salts     150      g./l.                                             sodium cyanide     30       mg./l.                                            formaldehyde (37%) 15       ml./l.                                            wetting agent      1        ml.                                               sodium hydroxide   to pH 13                                                   water (to make)                                                               ______________________________________                                    

The conductor pattern is built up to the desired thickness, the mask isremoved, and the finished printed circuit board is post cured for 30minutes at about 320° F.

EXAMPLE 2

The procedure of Example 1 is repeated, substituting for the firstmedium, one comprising

    ______________________________________                                        Cu.sub.2 O            15     g./l.                                            hydrochloric acid (37%)                                                                             300    ml./l.                                           NaH.sub.2 PO.sub.4.H.sub.2 O                                                                        30     g./l.                                            water (to make)                                                               ______________________________________                                    

Substantially the same results are obtained.

EXAMPLE 3

The procedure of Example 1 is repeated, substituting for the firstmedium, one comprising:

    ______________________________________                                        CuSO.sub.4.5H.sub.2 O                                                                              25     g./l.                                             NiSO.sub.4.6H.sub.2 O                                                                              25     g./l.                                             NH.sub.4 OH (conc.)  200    ml./l.                                            water (to make).                                                              ______________________________________                                    

EXAMPLE 4

The procedure of Example 1 is repeated, substituting for the firstmedium, one comprising:

    ______________________________________                                        CuSO.sub.4.5H.sub.2 O                                                                              25     g./l.                                             NiSO.sub.4.5H.sub.2 O                                                                              25     g./l.                                             NH.sub.4 OH (conc.)  200    ml./l.                                            hydroquinone         2      g./l.                                             water (to make).                                                              ______________________________________                                    

EXAMPLE 5

The procedure of Example 1 is repeated, substituting for the firstmedium, one comprising:

    ______________________________________                                        FeSO.sub.4           20     g./l.                                             NH.sub.4 OH (conc.)  200    ml./l.                                            water (to make).                                                              ______________________________________                                    

EXAMPLES 6-12

The procedure of Example 1 is repeated, substituting, respectively, forthe first medium, those comprising:

EXAMPLE 6

    ______________________________________                                         NiSO.sub.4.6H.sub.2 O                                                                             25     g./l.                                             NH.sub.4 OH (conc.)  200    ml./l.                                            water (to make).                                                              ______________________________________                                    

EXAMPLE 7

    ______________________________________                                        CuSO.sub.4.5H.sub.2 O                                                                              25     g./l.                                             NH.sub.4 OH (conc.)  200    ml./l.                                            water (to make).                                                              ______________________________________                                    

EXAMPLE 8

    ______________________________________                                        Co(C.sub.2 H.sub.3 O.sub.2).sub.2.4H.sub.2 O                                                       25     g./l.                                             NH.sub.4 OH (conc.)  200    ml./l.                                            water (to make).                                                              ______________________________________                                    

EXAMPLE 9

    ______________________________________                                        CuCl                  10     g./l.                                            HCl (conc.)           300    ml./l.                                           stannous chloride.2H.sub.2 O                                                                        30     g./l.                                            water (to make).                                                              ______________________________________                                    

EXAMPLE 10

    ______________________________________                                        CuSO.sub.4.5H.sub.2 O 25    g./l.                                             triethanolamine       40    g./l.                                             water (to make).                                                              ______________________________________                                    

EXAMPLE 11

The procedure of Example 1 is repeated, but substituting for water asthe second medium, 1 g. of sodium borohydride in 1000 ml. of water (pH9.5-10.0).

EXAMPLE 12

A combination of metals comprises

    ______________________________________                                        CuSO.sub.4.5H.sub.2 O 10    g./l.                                             NiSO.sub.4.6H.sub.2 O 10    g./l.                                             Benzyl trimethyl                                                              ammonium                                                                      chloride              10    g./l.                                             water (to make).                                                              ______________________________________                                    

Substantially the same results are obtained.

EXAMPLE 13

This example illustrates the use of two separate reducers in the processof the present invention.

A disc of acrylonitrile-butadiene-styrene terpolymer (ABS) is immersedfor 5 minutes in an alkaline cleaner at 65° C. Oakite 90 or Altrex areillustrative of suitable commercial materials and trisodium phosphate isalso suitable. The cleaned ABS disc is rinsed for 5 minutes in water,then the surface is rendered permanently polarized and wettable byimmersion for 1 minute in a solution of 250 ml./l. of methyl ethylketone in water, rinsing in water for 3-5 minutes, immersing in asolution of 370 g./l. of chromatic acid, 185 ml./l. of concentratedsulfuric acid, 0.5 g./l. of fluorinated hydrocarbon wetting agent (3MCompany, FC-95), balance water, for about 5-8 minutes, rinsing in waterfor 1 minute, immersing in a solution of hydroxyl-amine hydrochloride,20 g./l., hydrochloric acid (37%), 200 ml./l., balance water, for about2 minutes and rinsing thoroughly in water for 5 minutes.

Then the activated disc is immersed in a solution comprising

    ______________________________________                                        CuSO.sub.4.5H.sub.2 O                                                                              25     g./l.                                             NiSO.sub.4.6H.sub.2 O                                                                              25     g./l.                                             NH.sub.4 OH (conc.)  200    ml./l.                                            water (to make)                                                               ______________________________________                                    

The so-treated disc is rinsed gently with water for 1 minute at pH (5-8)and about 15°-20° C., to decrease the solubility of the copper andnickel compounds and to remove the excess.

The disc is next immersed in a first reducer solution comprising

    ______________________________________                                        formaldehyde (37%)                                                                             100 ml./l.                                                   water (to make)                                                               ______________________________________                                    

for 1 minute, and the excess first solution is drained off for about15-30 seconds.

Then the disc is immersed for 5 minutes in a second reducer solutioncomprising

    ______________________________________                                        sodium borohydride  1 g./l.                                                   sodium hydroxide (50%)                                                                            2 ml./l.                                                  deionized water (to make)                                                     ______________________________________                                    

(The second reducer is made by adding the NaBH₄ to water and mixing for5 minutes, then adding the NaOH and mixing for 5 minutes longer).

The disc which now has catalytic surface sites of elemental copper andnickel is rinsed for 2 minutes with water (pH 5.0-8.0).

The sensitized disc is transferred to an electroless copper depositionbath of the formula:

    ______________________________________                                        CuSO.sub.4.5H.sub.2 O                                                                            30       g./l.                                             Rochelle salts     150      g./l.                                             sodium cyanide     30       mg./l.                                            formaldehyde (37%) 15       ml./l.                                            wetting agent      1        ml.                                               sodium hydroxide   to pH 13                                                   water (to make)                                                               ______________________________________                                    

Electroless copper builds up to the desired thickness.

EXAMPLES 14-24

The procedure of Example 13 is repeated, substituting for thesensitizing medium described, the media used in Examples 1, 2, 4-10 and12, respectively. ABS discs metallized with adherent electroless copperare obtained.

The invention is not limited to printed circuit boards or to thespecific steps and methods described. Moreover, electroless metal bathsfor depositing any other conventional metal than copper, e.g., nickel,cobalt, silver, gold, and the like, can be used. Other anions, besidesthose exemplified, e.g., phosphates, bromides, fluorides, fluoborates,etc., can be used with the compound containing an element which isultimately the catalytic agent.

We claim:
 1. A process for sensitizing the surface of a non-metallicarticle for the deposition of an adherent metal from an electrolessmetal deposition solution in contact therewith, comprising the stepsof:(a) adsorbing on the surface or selected areas of the surface of saidarticle, copper (I) ion compound by contacting said surface with a firstmedium comprising an aqueous solution containing a compound of saidcopper (I) ion compound and a compound of stannous tin; (b) next rinsingsaid article with an aqueous second medium to decrease the solubility ofthe copper (I) ion compound adsorbed on said surface, while at the sametime, removing from said surface with said second medium unadsorbedcopper (I) ion compound carried over from said first medium; and (c)treating said article with at least one reducing agent to render saidadsorbed copper (I) ion compound catalytically active for theelectroless deposition of metal.
 2. The process of claim 1, wherein saidfirst medium is acidic.
 3. The process of claim 1, wherein said reducingagent is a member of the group consisting of borohydrides, amine boranesand formaldehyde.
 4. The process of claim 1, wherein the treatment witha reducing agent is carried out in two separate stages, employing twoseparate reducing agents.
 5. The process of claim 4, wherein thereducing agent in the first separate stage is formaldehyde, and thereducing agent in the second separate stage is a borohydride.
 6. Aprocess for metallizing the surface of a non-metallic article by thedeposition of an adherent metal from an electroless metal depositionsolution in contact therewith, comprising the steps of:(a) adsorbing onthe surface or selected areas of the surface of said article, copper (I)ion compound by contacting said surface with a first medium comprisingan aqueous solution containing a compound of said copper (I) ioncompound and a compound of stannous tin; (b) next rinsing said articlewith an aqueous second medium to decrease the solubility of the copper(I) ion compound adsorbed on said surface, while at the same timeremoving from said surface with said second medium unadsorbed copper (I)ion compound carried over from said first medium; (c) treating saidarticle with at least one reducing agent to render said adsorbed copper(I) ion compound catalytically active for the electroless deposition ofmetal; and (d) contacting said catalytically active article with anelectroless metal deposition bath, to deposit metal thereon.
 7. Theprocess of claim 6, wherein said first medium is acidic.
 8. The processof claim 6, wherein said reducing agent is a member of the groupconsisting of borohydrides, amine boranes and formaldehyde.
 9. Theprocess of claim 6, wherein the treatment with a reducing agent iscarried out in two separate stages, employing two separate reducingagents.
 10. The process of claim 9, wherein the reducing agent in thefirst separate stage is formaldehyde, and the reducing agent in thesecond separate stage is a borohydride.